Why are the assigned Site Group for a user being reset to default when using single sign-on (SSO) ?

When creating or logging in as a Site User, the mapping to a Site Group can be defined for the user to determine which Site Group the user should be placed within.  
While it is possible in Pressero to assign a user to multiple Site Groups, the mapping for single sign-on (SSO) only enables you to map the Site User to a single Site Group.  If this user's Site Groups are altered to be assigned to multiple Site Groups within Pressero, the next time the user logs in via single sign-on (SSO) this will be reset to the default Site Group assigned at the time of login instead.